FIG. 14 is a block diagram illustrating a circuit structure of a portable telephone in general use. In FIG. 14, reference numeral 101 designates an antenna, numeral 102 designates a duplexer (hereinafter referred to as a DUP), numeral 103 designates a low noise amplifier (hereinafter referred to as an LNA) for amplifying an input signal, and numeral 104 designates a demodulator for demodulating the signal amplified by the LNA 103. Reference numeral 105 designates a frequency synthesizer, numeral 106 designates an analog-to-digital converter (hereinafter referred to as an ADC) for converting an analog signal output from the demodulator 104 into a digital signal, and numeral 107 designates a temperature compensated crystal oscillator (hereinafter referred to as a TCXO). Reference numeral 108 designates a signal processor, numeral 109 designates a digital signal processor (hereinafter referred to as a DSP), numeral 110 designates an ADC for converting an analog signal from a microphone 111 into a digital signal and outputting the digital signal to the DSP 109, numeral 112 designates a digital-to-analog converter (hereinafter referred to as a DAC) for converting a digital signal output from the DSP 109 into an analog signal and outputting the analog signal to a speaker 113, numeral 114 designates a controller, numeral 115 designates a display, such as a liquid crystal display, numeral 116 designates a console provided with input buttons, such as numeric keys, and numeral 117 designates a memory. Reference numeral 118 designates a DAC for converting a digital signal output from the signal processor into an analog signal, numeral 119 designates an orthogonal modulator for orthogonally modulating a signal output from the DAC 118, and numeral 120 designates an amplifier for amplifying a signal output from the orthogonal modulator 119.
In the transmitter side amplifier 120 (high power amplifier) in the portable telephone thus constructed, a bipolar transistor is usually used as an amplifier element to amplify a microwave signal output from the orthogonal modulator 119 with high efficiency. Besides, bipolar transistors are used as amplifier elements in a high power amplifier for transmission from a base station and in a high power amplifier mounted on a communication satellite.
There are only two operating modes of a bipolar transistor, i.e., a base voltage (V.sub.B) constant operating mode and a base current (I.sub.B) constant operating mode. These modes have their respective merits and demerits.
FIG. 15 shows measurement results of I/O characteristics of an HBT (Heterojunction Bipolar Transistor) with a single emitter having a size of 2.times.20 .mu.m.sup.2. The bias conditions are V.sub.CE =8 V and I.sub.B =1.75 mA, that is, V.sub.CE and I.sub.B are constant. The measuring frequency is 18 GHz. Here, "constant" means that the time average of V.sub.CE (I.sub.) is constant.
As shown in FIG. 15, when P.sub.in is higher than 11 dBm, the base voltage (V.sub.BE) decreases. In FIG. 15, V.sub.BE is the time average of the base voltage V.sub.BE.
In FIG. 15, when P.sub.in is 14 dBm, providing the highest efficiency, V.sub.BE is already as low as 1.062 V. Since the ON voltage of the HBT is about 1.30 V, the change of V.sub.BE with the passage of time is as shown in FIG. 16(a). Since the collector current (I.sub.CE) does not flow at a voltage lower than the ON voltage, the change of I.sub.CE with the passage of time is as shown in FIG. 16(b). That is, I.sub.CE flows like a pulse only when V.sub.BE exceeds the ON voltage (about 1.30 V). Therefore, class "C" operation of the transistor is realized, resulting in a high efficiency operation of the transistor. When P.sub.in =-4 dBm, V.sub.BE =1.318 V and I.sub.CE =11.65 mA. Therefore, when the input amplitude is small, the transistor performs in class "A" operation. With an increase in the input amplitude, the operation class changes automatically from "B" (P.sub.in =12 dBm) to "C" (P.sub.in .gtoreq.13 dBm).
A description is given of the reason why the bias point changes from "A" at the initial setting, through "B" to "C". In the example mentioned above, the measuring conditions are V.sub.CE =8 V (constant) and I.sub.B =1.75 mA (constant). Here, "constant" means that the time average of V.sub.CE (I.sub.B) is constant. However, since it is unthinkable that V.sub.CE vary with the input amplitude, the bias point cannot get move off the line of V.sub.CE =8 V. In addition, "I.sub.B is constant" means ##EQU1##
FIG. 17 shows load curves on the assumption that the bias point Q does not move with an increase in the input amplitude. Hereinafter, an investigation is made into a case where the input amplitude is small (P.sub.in1), and a case where the input amplitude is large, and the output current amplitude is already restricted by the x axis (I.sub.CE =0 mA) on the high V.sub.CE side and moves along the x axis (P.sub.in2 ). FIGS. 18(a) and 18(b) show changes of I.sub.B and I.sub.CE with the passage of time, respectively, in the case of P.sub.in1. FIGS. 19(a) and 19(b) show changes of I.sub.B and I.sub.CE with the passage of time, respectively, in the case of P.sub.in2.
The fact that the bias point Q does not move means that the center of the amplitude of I.sub.B is on I.sub.B =1.75 mA. In order to satisfy formula (1), hatched regions A and B in FIG. 18(a) must have the relationship of A=B, and hatched regions A' and B' in FIG. 19(a) must have the relationship of A'=B'. In FIG. 18(a), A=B is realized because I.sub.B does not become 0. However, in FIG. 19(a) where the input amplitude is large, since I.sub.B cannot take a negative value, A'&lt;B' results, so that formula (1) is not satisfied. So, in order to increase the input amplitude with formula (1) being satisfied, the bias point Q moves downward along the line of V.sub.CE =8 V. FIG. 20 shows load curves in the case where the bias point Q moves downward. As shown in FIG. 20, with an increase in the input amplitude (P.sub.in1 .fwdarw.P.sub.in2 .fwdarw.P.sub.in3), the bias point moves downward (Q.sub.1 .fwdarw.Q.sub.2 .fwdarw.Q.sub.3) to satisfy formula (1). FIGS. 21(a) and 21(b), 21(c) and 21(d), and 21(e) and 21(f), respectively show changes of I.sub.B and I.sub.CE with time at the input levels P.sub.in1, P.sub.in2, and P.sub.in3, respectively. In order to satisfy formula (1), the area of the hatched region in the figure must always be constant (I.sub.B (=1.75 mA).times..DELTA.t). In order to make the area constant, an increment in the area due to the increase in the amplitude is canceled, whereby the center value of the amplitude is lowered. In other words, the bias point Q moves downward. As shown in FIGS. 16(a) and 16(b), under a condition where the efficiency attains the maximum value, since V.sub.BE (=1.06 V) is lower than the ON voltage, it is thinkable that the transistor is in the state of P.sub.in3, so that the class "C" operation of the transistor is realized, resulting in a high-efficiency operation of the transistor. This result is attributed to the fact that the theoretical efficiency attained in the class "C" operation of the bipolar transistor is 100% whereas the theoretical efficiency attained in the class "AB" operation is about 70% at best.
As described above, when a bipolar transistor is operated by a bias circuit with constant I.sub.B since V.sub.B drops, the bias point, i.e., the operating class, changes (for example, "AB".fwdarw."B".fwdarw."C"), whereby a high efficiency is achieved while maintaining a linear of gain.
However, the reduction of V.sub.B with the increase in P.sub.in causes a disadvantage that P.sub.out does not increase. For example, when a bipolar transistor used as an amplifier in a portable telephone is operated by a bias circuit with constant I.sub.B, an output power needed when the telephone is used in a place distant from the base station is not sufficiently obtained. In order to increase P.sub.out (I.sub.C) in the I.sub.B constant operating mode, I.sub.B must be increased. However, since to increase I.sub.B is to increase the initial bias current (I.sub.idle) in the state with no input signal (P.sub.in), the efficiency decreases in the region where P.sub.in is low. In addition, when I.sub.idle is high, the power consumption increases and the junction temperature increases.
On the other hand, in the V.sub.B constant operating mode, since I.sub.B and I.sub.C (collector current) increase with an increase in the RF input (P.sub.in), the RF output (P.sub.out) increases, whereby a P.sub.out higher than a certain value is secured even though the initial bias current (I.sub.idle) is lowered. However, since V.sub.B is constant, the bias point, i.e., the operating class, is fixed. As a result, a high efficiency cannot be realized while maintaining linearity of gain.
As described above, in the conventional bias circuit for operating a bipolar transistor, the bipolar transistor is operated with a constant base voltage or a constant base current. However, in a bias circuit with a constant base voltage, a high efficiency cannot be realized while maintaining linearity of gain. On the other hand, in the bias circuit with a constant base current, a sufficient output power cannot be obtained when the input amplitude is large.